The present invention relates to a process for producing a contact lens and, more particularly, to a process for producing a hard contact lens that has good dimensional stability and high oxygen permeability and which can be worn for a prolonged period.
The use of contact lenses has expanded at rapid rates because of the many advantages they have over conventional spectacles, such as lightness, unobtrusiveness in the wearer's activity, good aesthetic appeal, and sufficiently good fit to the cornea to ensure a wide corrected field of vision. Most of the hard contact lenses available today are formed of plastic materials. Since the principal component of hard contact lenses is a methyl methacrylate polymer, they have good optical performance but they provide for such a low degree of oxygen permeability that they will cause problems such as spectacles blur and corneal edema if they are worn longer than the indicated period. In addition, the "hard" contact lenses do not have high affinity for the cornea and will often cause excessive tear flow or discomfort to the wearer.
In order to eliminate these problems of hard contact lenses, soft contact lenses that are based on hydrophilic hydroxyethyl methacrylate polymers have been developed. Compared with hard contact lenses based on methyl methacrylate polymers, soft contact lenses cause less discomfort to the wearer, have good affinity for the cornea and exhibit improved oxygen permeability. However, hydrated soft contact lenses are easily contaminated by bacteria and must be disinfected by boiling at least once a day. In addition to this cumbersomeness in use, soft contact lenses do not have sufficient mechanical strength to warrant their use for many years. Furthermore, even if the cornea of the wearer is damaged by the contact lens, he will not be aware of the damage until it has caused a serious problem.
Silicone rubber is one of the plastic materials that have the highest degree of oxygen permeability, and increasing attention is being paid to the fabrication of contact lenses from silicone rubber. These new types of soft contact lenses are still disadvantageous in that they have to be formed by a casting process and that they are hydrophobic and do not have high affinity for the eye. Under these circumstances, efforts are being made to produce contact lenses from siloxanyalkyl ester monomers.
The siloxanyalkyl ester monomers that are usable as materials from which contact lenses can be formed are their esters with acrylic or methacrylic acid (Japanese Patent Public Disclosure No. 132725/1983). However, these vinyl monomers are not highly polymerizable and active efforts are being made in order to maximize their polymerization by adding large amounts of other polymerizable vinyl monomers, but it still has been very difficult to prepare polymers having no residual unreacted monomers. Unreacted monomers which remain in a contact lens may cause inflammations in the cornea of the wearer and their content must be lowered to a minimum level. This requirement could be met by performing thermal polymerization in the presence of a catalyst but the amount of the catalyst that can be used is limited because if it is used in a large amount, it will be decomposed in the early period of polymerization and tiny pieces of the decomposed catalyst will remain in the product contact lens. The residual catalyst fragments are also highly deleterious to the dimensional stability of the contact lens. Therefore, in the production of contact lenses by catalyst-aided thermal polymerization of siloxanylalkyl ester monomers, the elimination of any unreacted monomers and decomposed catalyst chips is important not only from a medical viewpoint but also for the purpose of attaining good lens performance.
If a mixture of multiple monomer components having different degrees of polymerizability is subjected to thermal polymerization by the static method, the difference in the rates of polymerization of the individual components will cause bubbles in the polymer or lead to the production of a shrunken polymer.
Another problem that has been encountered in the prior art is that a polymer with decreased oxygen permeability results if prolonged reaction is performed with a view to attaining a desired polymer hardness.